1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method. More specifically, the present invention relates to, for example, a color separation technology for use in printing with special-color ink.
2. Description of the Related Art
In order to print images with an outputting device including a printer, color separation is required to convert RGB image data into CMYK (cyan, magenta, yellow, and black) data representing an amount of jetted ink used in the printer. A color separation method typically uses a table, that is, a lookup table (LUT), in which input RGB values have a one-to-one correspondence with output CMYK values.
In addition, when a large number of identical images are printed in offset printing or the like, special-color printing with ink having five colors or more including at least one special color in addition to CMYK colors is adopted in order to accurately reproduce brighter colors in an original image.
Generally, the number of patches necessary for creating an LUT is represented by the following equation.
[Formula 1]P=nm  (1)where n denotes the number of grids of one-color ink and m denotes the number of kinds of ink.
When four-color ink is used and the number of grids is eight, 84=4096 patches are required. In contrast, when special colors are added to increase the number of kinds of ink (for example, six-color ink having CMYK colors, orange (O), and green (G) is used), the number of patches necessary to create an LUT is increased to 86=262,144, thus requiring an enormous cost for output and color measurement.
Inversely, when the six-color ink is used to create an LUT including 4,096 sample patches, as in the case where the four-color ink is used and the number of grids is eight, four grids are used for each color in the LUT, thus reducing the accuracy of color reproduction.
One color separation method for special-color printing with ink having five or more colors is disclosed in Japanese Unexamined Patent Application Publication No. 2001-136401. In this method, an LUT is divided for every hue and a plurality of LUTs having a total of four colors is prepared to switch the LUTs for every input data, instead of allowing for all combinations of the used ink. Each LUT has two arbitrary colors having adjacent hues, a special color sandwiched therebetween, and black (K).
In addition, another color separation method is disclosed in Japanese Unexamined Patent Application Publication No. 06-326861. In this method, color reproduction prediction is performed by using a printer model and input RGB values and output CMYK values are converted into Lab values in a color space independent of a device, such as a CIE Lab uniform color space, to determine color separation values so as to make smaller differences between the Lab values converted from the input RGB values and those converted from the output CMYK values.
However, in the technology disclosed in Japanese Unexamined Patent Application Publication No. 2001-136401, since an LUT is created for every hue and the LUTs are switched for every Lab value of the input data, there is a problem in that only up to four-color ink is used for one input value, despite of using the ink having five or more colors.
In the technology disclosed in Japanese Unexamined Patent Application Publication No. 06-326861, a printer model is used to perform color reproduction prediction for color separation. Since the color separation is performed for every pixel without allowing for image information, that is, the relationship between the value of a target pixel and the values of neighborhood pixels, there is a problem in that, when the differences among the color separation results of pixels having approximate RGB values are widened due to the errors of the color reproduction prediction, areas where the values smoothly vary in the input image are reproduced in significantly different colors in the output image, causing noise or pseudo outlines.